Q-plate enabled spectrally diverse orbital-angular-momentum conversion for stimulated emission depletion microscopy

TLDR

Spin to orbital angular momentum (OAM) conversion using a q‑plate has recently attracted attention as a convenient way to generate OAM beams. We demonstrate that the dispersive properties of a q=1/2 plate, specifically its group‑index difference Δng for ordinary and extraordinary polarizations, can be tuned to realize a single‑aperture, alignment‑tolerant STED nanoscopy system with versatile control over color combinations and laser bandwidths. Point‑spread‑function measurements show that the system achieves high‑throughput (>89 %) and high‑purity donut beams (extinction ratios up to −18.75 dB, ~1 % residual light) across the visible spectrum, enables dual‑color STED illumination, supports wide bandwidths up to 19 nm (ultrashort pulses ~50 fs), and allows switching between color settings by simply adjusting the q‑plate bias without altering alignment, thereby enabling alignment‑free, spectrally diverse multiplexed nanoscale imaging.

Abstract

Spin to orbital angular momentum (OAM) conversion using a device known as a q-plate has gained recent attention as a convenient means of creating OAM beams. We show that the dispersive properties of a q=1/2 plate, specifically its group index difference Δng for ordinary and extraordinary polarization light, can be tuned for achieving single-aperture, alignment-tolerant stimulated emission depletion (STED) nanoscopy with versatile control over the color combinations as well as laser bandwidths. Point spread function measurements reveal the ability to achieve single-aperture STED illumination systems with high throughput (transmission >89%) and purity (donut beam extinction ratios as high as |−18.75|  dB, i.e., ∼1% residual light in the dark center of the donut beam) for a variety of color combinations covering the entire visible spectrum, hence addressing several of the fluorescent dyes of interest in STED microscopy. In addition, we demonstrate dual-color STED illumination that would enable multiplexed imaging modalities as well as schemes that could use wide bandwidths up to 19 nm (and hence ultrashort pulses down to ∼50  fs). Switching between any of these color settings only involves changing the bias of the q-plate that does not alter the alignment of the system, hence potentially facilitating alignment-free, spectrally diverse multiplexed nanoscale imaging.